The invention relates to a process and to a device for measuring the thickness of transparent materials. More particularly, but not exclusively, the invention concerns the thickness measurement of glass materials and, even more precisely, the thickness measurement of flat glass, in particular float glass.
The general quality requirements demanded by customers and the savings which can be made by keeping to the bottom of the thickness tolerance range require very rigorous monitoring of thickness in the mass production of flat glass.
Of the techniques normally used for measuring thickness, the most precise methods which can be used in transparent media are optical methods. Among these, interferometric techniques, previously limited to laboratory measurements, have progressively found industrial applications.
For example, document FR-A-2 435 019 proposes a technique for measuring the thickness of a thin film which consists in exposing the thin film to infrared light spectroscopically split by rapid scanning over a range of wavelengths which is predetermined as a function of the nature of the film so as to create a spectrum of interference fringes between the reflected rays, the extreme points of which are determined. The technique is limited to thicknesses necessarily smaller than 30 .mu.m. It consists in counting the interference fringes of rays reflected by the surfaces of the film. Such a method cannot be used for measuring the thickness of flat glass on a float glass production line whose thickness varies from less than 1 mm to 2 cm.
Another document, WO 95/22740, describes an interference method for determining the wall thickness of bottles during their manufacture.
The process is characterized in that a light beam with modulated optical frequency is emitted, in that two light beams or rays, reflected by each of the surfaces of a wall of a material, are received, in that interference is created between them and in that the path difference .delta. of the interference signal is determined. A laser diode is used as the illumination source, and this is modulated by modulation of the optical frequency of the beam. Of the rays scattered by the two walls, two parallel rays are selected. The device of the invention makes it possible to take measurements 0.3 msec apart on each sensor. It is thus possible to explore every millimetre of the periphery of a bottle in rotation.
This technique, in which measurements are taken using isolated rays scattered by the surfaces, requires relatively powerful lasers (&gt;30 mW), which may present drawbacks. It will be difficult to use the same method with parallel reflected beams because of the prismaticity of the support and, in particular, of float glass which is always prismatic in the edge zones.
Although providing good precision on the absolute thickness measurement, the method of WO 95/22740 is less well suited to following local thickness variations. This type of measurement is, however, very important for detecting the drifts in the nominal thickness of flat glass on its production line as early as possible. Furthermore, the method of WO 95/22740 does not make it possible to measure thicknesses smaller than 0.7 mm.